In recent years, carbon dioxide, a typical global warming gas, has been discharged from power plants, cement plants, steel plants, chemical plants, etc. In terms of the prevention of global warming, there an urgent need to develop a high-efficiency carbon dioxide recovery technology. In addition, carbon dioxide is also present in natural gases containing methane as a main component, and, in terms of the prevention of pipeline corrosion, carbon dioxide needs to be recovered and removed from methane in addition, in an integrated coal gasification combined cycle, an attempt has been made in which finely pulverized coal is pyrolyzed and gasified to produce, through a shift reaction, a gas containing hydrogen and carbon dioxide as main components, and then carbon dioxide is separated to use the resulting hydrogen-rich gas as a fuel gas.
Conventionally, as methods for recovering carbon dioxide, technologies including amine absorption and like chemical absorption methods, pressure swing adsorption (PSA) and like physical adsorption methods, and the like have been used. However, because the energy consumption for the regeneration of the absorbing liquid or adsorbent is high, the development of a higher-efficiency recovery method has been expected.
Incidentally, a membrane separation method using a zeolite membrane allows for continuous operation and does not require the regeneration of an absorbing liquid or adsorbent, and thus is increasingly expected to serve as a high-efficiency carbon dioxide recovery technology.
For example, Nonpatent Document 1 shows an example in which carbon dioxide or nitrogen is separated from a natural gas containing methane as a main component by membrane separation using a zeolite membrane.
Meanwhile, in a membrane separation method, the pressure difference through a zeolite membrane is the driving force for gas permeation. Therefore, the carbon dioxide concentration on the non-permeate side of the zeolite membrane decreases with the removal and recovery of carbon dioxide, and thus the reality is that the separation efficiency decreases in a region of low concentration of carbon dioxide.
For the above reasons, Patent Document 1, for example, proposes a hybrid process of a zeolite membrane for separation and a pressure swing adsorption (PSA) method, in which for the separation and recovery of carbon dioxide from an exhaust gas, carbon dioxide is first primarily concentrated in a membrane separation apparatus, and further secondarily concentrated in an adsorption separation apparatus, thereby recovering high-concentration carbon dioxide.